Glycoproteins IIB and IIIa are platelet surface glycoproteins which mediate aggregation of one platelet to another. During cell activation, GPIIb and GPIIIa as one cell bind fibrinogen and contact GPIIb and GPIIIa on another cell to form an adherent complex. The overall goal of this project is to investigate the structural features of these two proteins which permit them to function as cytoadhesins. cDNA libraries from HEL cells and an endothelial cell hybrid, which expresses GPIIb- and GPIIIa-like proteins, will be screened for clones encoding GPIIb- and GPIIIa- like proteins using both immunological and nucleotide probes. The cDNA clones for each protein will be subcloned into M13 and the nucleotide sequence determined. The sequence of HEL cell GPIIb- and GPIIIa-like proteins which form a calcium-dependent complex and bind complex-specific monoclonal antibodies, will be compared with the endothelial cell proteins, which do not dissociate in the absence of calcium or bind complex-specific monoclonal antibodies, to identify molecular domains involved in calcium binding, fibrinogen binding, and complex formation. cDNA probes will be used to identify genomic DNA for GPIIb and GPIIIa. The genetic defect in patients with Glanzmann's thrombasthenia, who are deficient in or have an abnormal GPIIb and GPIIIa, will be determined. Finally, we will initiate studies to express GPIIb and GPIIIa in eukaryotic cells with a long range goal of starting in vitro mutagenesis studies to further probe the structure-function relationships of these two proteins.